DBIx-Class-DeploymentHandler-0.002215

NAME

DBIx::Class::DeploymentHandler::HandlesVersioning - Interface for version methods

DESCRIPTION

Typically a VersionHandler will take a to_version and yeild an iterator of version sets.

Typically a call to a VersionHandler's "next_version_set" with a db_version of 1 and a to_version of 5 will iterate over something like the following:

[1, 2]
[2, 3]
[3, 4]
[4, 5]
undef

or maybe just

[1, 5]
undef

Really how the version sets are arranged is up to the VersionHandler being used.

In some cases users will not want versions to have inherent "previous versions," which is why the version set is an ArrayRef. In those cases the user should opt to returning merely the version that the database is being upgraded to in each step.

One idea that has been suggested to me has been to have a form of dependency management of the database "versions." In this case the versions are actually more like features that may or may not be applied. For example, one might start with version 1 and have a feature (version) users.

Each feature might require that the database be upgraded to another version first. If one were to implement a system like this, here is how the VersionHandler's "next_version_set" might look.

to_version = "users", db_version = 1
[3]
[5]
["users"]
undef

So what just happened there is that users depends on version 5, which depends on version 3, which depends on version 1, which is already installed. To be clear, the reason we use single versions instead of version pairs is because there is no inherent order for this type of database upgraded.

Downgrades

For the typical case downgrades should be easy for users to perform and understand. That means that with the first two examples given above we can use the "previous_version_set" iterator to yeild the following:

db_version = 5, to_version=1
[5, 4]
[4, 3]
[3, 2]
[2, 1]
undef

or maybe just

[5, 1]
undef

Note that we do not swap the version number order. This allows us to remain consistent in our version set abstraction, since a version set really just describes a version change, and not necesarily a defined progression.